Cesium and strontium extraction using a mixed extractant solvent including crown ether and calixarene extractants

ABSTRACT

A mixed extractant solvent including calix[4]arene-bis-(tert-octylbenzo)-crown-6 (“BOBCalixC6”), 4′,4′,(5′)-di-(t-butyldicyclo-hexano)-18-crown-6 (“DtBu18C6”), and at least one modifier dissolved in a diluent. The mixed extractant solvent may be used to remove cesium and strontium from an acidic solution. The DtBu18C6 may be present from approximately 0.01 M to approximately 0.4M, such as from approximately 0.086 M to approximately 0.108 M. The modifier may be 1-(2,2,3,3-tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol (“Cs-7SB”) and may be present from approximately 0.01M to approximately 0.8M. In one embodiment, the mixed extractant solvent includes approximately 0.15M DtBu18C6, approximately 0.007M BOBCalixC6, and approximately 0.75M Cs-7SB modifier dissolved in an isoparaffinic hydrocarbon diluent. The mixed extractant solvent may form an organic phase in an extraction system that also includes an aqueous phase. Methods of extracting cesium and strontium as well as strontium alone are also disclosed.

GOVERNMENT RIGHTS

The United States Government has rights in the following inventionpursuant to Contract No. DE-AC07-99ID13727 between the U.S. Departmentof Energy and Bechtel BWXT Idaho, LLC.

FIELD OF THE INVENTION

The present invention relates to separating cesium and strontium from anacidic solution. More specifically, the present invention relates tosimultaneously separating cesium and strontium from the acidic solutionusing a mixed extractant solvent.

BACKGROUND OF THE INVENTION

Cesium-137, strontium-90, and actinides account for a significant amountof the radioactivity of liquid wastes, such as high level liquid wastesfrom nuclear fuel reprocessing. Cesium-137 and strontium-90 account forover 99.9% of the relative toxicity of the liquid waste once theactinides have been removed. Cesium-137 has a halflife (“t_(1/2)”) of 30years and strontium-90 has a t_(1/2) of 29 years. This liquid waste isextremely hazardous and expensive to dispose of. To increase safehandling of the majority of the liquid waste and to significantly reduceits storage and disposal cost, the liquid waste is separated into twoportions: one containing the majority of the radioactive components andone containing the bulk of the non-radioactive components. Removing theradioactive components allows the liquid waste to be decategorized anddisposed of in geological formations after vitrification. Currently,separate technologies are used to remove the actinides and fissionproducts from the liquid waste and, often times, separate processes areused to remove specific radionuclides, such as cesium and strontium.

The ability to remove and recover cesium and strontium from spentnuclear fuel waste represents a significant issue regarding short termheat loading in a geological repository. Cesium and strontium are majorheat generators in the liquid waste and produce gamma and betaradiation. Removing the cesium-137 and strontium-90 would enable theseradionuclides to be stored in a short-term waste facility, enablinglong-term storage facilities to store waste closer together byeliminating some of the heat load.

Liquid extraction, sorption, and coprecipitation methods have been usedto remove cesium or strontium from nuclear acidic waste solutions orrelated alkaline wastes. Numerous extractants have been identified thatextract cesium or strontium from alkaline solutions or acidic solutions.The extractants are typically separate solvents that are designed toremove one of these radionuclides. For instance, crown ether compoundsor calixarene crown ether compounds have been used to extract cesium.U.S. Pat. No. 6,174,503 to Moyer et al., U.S. Pat. No. 6,566,561 toBonnesen et al., Duchemin et al., Solvent Extr. And Ion Exch.,19(6):1037-1058 (2001), Leonard et al., Solvent Extr. And Ion Exch.,21(4):505-526 (2003), Leonard et al., Sep. Sci. and Technol.,36(5-6):743-766 (2001), White et al., Sep. Sci. and Technol.,38(12-13):2667-2683 (2003), and Norato et al., Sep. Sci. and Technol.,38(12-13):2647-2666 (2003) disclose extracting cesium from alkalinesolutions using calix[4]arene-crown ether compounds. Thecalix[4]arene-crown ether compounds and modifiers are dissolved in adiluent. The calixarene is calix[4]arene-bis-(tert-octylbenzo)-crown-6(“BOBCalixC6”). Strontium is removed from the alkaline solutions in aseparate process using monosodium titanate. One specific extractantincludes 0.007M BOBCalixC6, 0.750M1-(2,2,3,3-tetrafluoro-propoxy)-3-(4-sec-butylphenoy)-2-propanol(“Cs-7SB”), 0.003 M trioctylamine (“TOA”), and Isopar® L and is referredto herein as the caustic-side solvent extraction (“CSSX”) solvent. TheCSSX solvent provides a forward distribution ratio or coefficient forcesium (“D_(Cs)”) of 8.0 from a 1M nitric acid solution. Anotherspecific extractant includes 0.01M BOBCalixC6, 0.5M Cs-7SB, 0.001 M TOA,and Isopar® L.

U.S. Pat. No. 5,926,687 to Dozol et al. and Bonnesen et al.,“Development of Process Chemistry for the Removal of Cesium from AcidicNuclear Waste by Calix[4]arene-crown-6 ethers,” ACS Sym. Ser. 757(Calixarenes for Separations), 26-44 (2000) disclose extracting cesiumfrom acidic solutions using calix[4]arene-crown ether compounds. Whilethe tested calix[4]arene-crown ether compounds have high distributioncoefficients for cesium, they have low distribution coefficients forstrontium. Various calix[4]arene-crown ether compounds and modifierswere tested because the stability of the calix[4]arene-crown ethercompounds and modifiers differed in each of these solutions. In Dozol etal., Sep. Sci. and Technol., 34(6&7):877-909 (1999), monocrown orbiscrown calix[4]arenes in a 1,3 alternative cone conformation aredisclosed to remove cesium from acidic or alkaline solutions.

U.S. Pat. No. 5,888,398 to Dietz et al. discloses using an18-crown-6-ether to extract cesium from acidic solutions. The18-crown-6-ether selectively extracts cesium over other ions, such ashydrogen, aluminum, calcium, boron, and strontium.

U.S. Pat. Nos. 5,344,623 and 5,346,618 to Horwitz et al., U.S. Pat. No.6,511,603 to Dietz et al., Lamb et al., “Novel Solvent System for MetalIon Separation: Improved Solvent Extraction of Strontium(II) and Lead(II) as Dicyclohexano-18-crown-6 Complexes,” Sep. Sci. and Technol.,34(13):2583-2599 (1999), Chiarizia et al., “Composition of the OrganicPhase Species in the Synergistic Extraction of Sr²⁺ by Mixtures ofDi(2-Ethylhexyl)Alkylenediphosphonic Acids andDicyclohexano-18-crown-6,” Solvent Extr. And Ion Exch., 21(2):171-197(2003), and Tanigawa et al., Chem. Eng. J. 39:157-168 (1988) discloseextracting strontium from an acidic solution using crown ethers. Onespecific extractant includes a mixture of 0.15M4′,4′,(5′)-di-(t-butyldicyclo-hexano)-18-crown-6 (“DtBu18C6”) and 1.2Mtri-n-butyl phosphate (“TBP”) in Isopar®L and is referred to herein asthe strontium extraction (“SREX”) solvent, as described in Horowitz etal., Solvent Extr. And Ion Exch., 9(1):1-25 (1991). The SREX solventprovides a distribution ratio or coefficient for strontium (“D_(Sr)”) of0.7 from a 1M nitric acid solution.

However, using separate extractants to remove the cesium and strontiumis disadvantageous in regard to environmental concerns, safety,simplicity and effectiveness of processing, and undesirable generationof secondary waste.

Methods of extracting both cesium and strontium have also beendisclosed. In U.S. Pat. No. 4,749,518 to Davis et al., cesium isextracted from acidified nuclear waste with bis 4,4′(5)[1-hydroxy-2-ethylhexyl]benzo 18-crown-6 and a cation exchanger. Thestrontium is then extracted using bis 4,4′(5′)[1-hydroxyheptyl]cyclohexo 18-crown-6 and a cation exchanger. In U.S.Pat. No. 5,393,892 to Krakowiak et al., a method of removing alkalimetal and alkaline earth metals is disclosed. A solid inorganic supporthaving a ligand covalently bonded thereto is contacted with a solutionincluding the alkali metal and alkaline earth metals. The ligand is anoxygen donor macrocyclic polyether cryptand that selectively removes thealkali metal and alkaline earth metals. In U.S. Pat. No. 5,666,641 toAbney et al., a polymeric material including a polymer and a plasticizeris used to extract cesium and strontium. In U.S. Pat. No. 5,666,642 toHawthorne et al., metal dicarbollide ion complexes are used to removecesium and strontium from an aqueous fission product waste solution. Themetal dicarbollide ion complexes are used to sequentially remove thecesium and then the strontium. In Horwitz et al., International SolventExtraction Committee '96, “A Combined Cesium-StrontiumExtraction/Recovery Process,” p. 1285-1290 (1996), an extraction processusing di-t-butylcyclohexano-18-crown-6 and a macrocyclic polyether aredisclosed to simultaneously extract cesium and strontium.

In addition, a large scale demonstration of concurrent cesium andstrontium partitioning from defense-related nuclear waste was performedin Russia using a cobalt dicarbollide extraction process. In U.S. Pat.No. 6,270,737 to Zaitsev et al., a composition of a complex organoboroncompound and polyethylene glycol in an organofluorane diluent is used toextract cesium and strontium. The complex organoboron compound is ahalogenated cobalt dicarbollide. In U.S. Pat. No. 6,258,333 toRomanovskiy et al., a composition of a complex organoboron compound,polyethylene glycol, and a neutral organophosphorus compound in adiluent is used to simultaneously extract cesium and strontium. Thecomplex organoboron compound is a halogenated cobalt dicarbollide.However, this extraction process uses multiple chemicals and, therefore,adds significant volume to the waste volume produced by the extractionprocess.

It is desirable to develop an extraction process that simultaneouslyremoves or extracts cesium and strontium from an acidic solution. Such adevelopment would improve capacity of long-term storage facilities andreduce the need to create new storage facilities. In order to be usefulin large-scale processing applications, the solvent used in such anextraction process would desirably be highly selective, cost effective,produce reduced waste volume, and be relatively nonhazardous.

BRIEF SUMMARY OF THE INVENTION

The present invention comprises a mixed extractant solvent that includescalix[4]arene-bis-(tert-octylbenzo)-crown-6 (“BOBCalixC6”),4′,4′,(5′)-di-(t-butyldicyclo-hexano)-18-crown-6 (“DtBu18C6”), and atleast one modifier dissolved in a diluent. The BOBCalixC6 may be presentin the mixed extractant solvent from approximately 0.0025M toapproximately 0.025M. The DtBu18C6 may be present in the mixedextractant solvent from approximately 0.01M to approximately 0.5M, suchas from approximately 0.086 M to approximately 0.108 M. At least onemodifier may be1-(2,2,3,3-tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol(“Cs-7SB”), which may be present in the mixed extractant solvent fromapproximately 0.2M to approximately 1.0M. The diluent may be anisoparaffinic hydrocarbon. The mixed extractant solvent may furtherinclude trioctylamine, tri-n-butyl phosphate, or mixtures thereof. Inone embodiment, the mixed extractant solvent may include approximately0.15M DtBu18C6, approximately 0.007M BOBCalixC6, and approximately 0.75MCs-7SB modifier dissolved in an isoparaffinic hydrocarbon.

The present invention also comprises an extraction system that includesan organic phase and an aqueous phase. The organic phase includes themixed extractant solvent as referenced above. The aqueous phase includesan acidic solution, such as a dissolved spent nuclear fuel. The acidicsolution may have from approximately 0.01M to approximately 3M nitricacid. The acidic solution may also include cesium and strontium.

The present invention also comprises a method of separating cesium andstrontium from an acidic solution. The method includes providing anacidic solution that has cesium and strontium. The acidic solution iscontacted with a mixed extractant solvent as referenced above. Theacidic solution may include from approximately 0.01M to approximately 3Mnitric acid. After contacting the acidic solution with the mixedextractant solvent, a first organic phase and a first aqueous phase maybe formed. The cesium and strontium may be extracted into the firstorganic phase. The first organic phase and the first aqueous phase maybe separated, removing the cesium and strontium from the acidicsolution. The extraction of the cesium and strontium may be conducted ata temperature ranging from approximately 1° C. to approximately 40° C.,such as from approximately 10° C. to approximately 15° C.

The cesium, strontium, and mixed extractant solvent may be recovered bycontacting the first organic phase with a second aqueous phase. Thecesium and strontium may be extracted into the second aqueous phase,which is separated from the first organic phase. The cesium andstrontium may be recovered at a temperature ranging from approximately10° C. to approximately 60° C., such as from approximately 20° C. toapproximately 40° C.

The present invention also comprises a method of extracting strontiumfrom an acidic solution. The acidic solution includes strontium and iscontacted with a solvent that includes DtBu18C6, Cs-7SB, and anisoparaffinic hydrocarbon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming that which is regarded as the present invention,the advantages of this invention may be more readily ascertained fromthe following description of the invention when read in conjunction withthe accompanying drawings in which:

FIG. 1 shows nitric acid dependencies for SREX at approximately 24° C.;

FIG. 2 show nitric acid dependencies for CSSX at approximately 24° C.;

FIG. 3 shows the D_(Cs), and D_(Sr) in 1M HNO₃ with variedconcentrations of DtBu18C6 in the CSSX solvent at 24° C.;

FIG. 4 shows the temperature dependence of an embodiment of a mixedextractant solvent that includes DtBu18C6, BOBCalixC6, and Cs-7SB;

FIG. 5 shows the nitrate dependence of the SREX solvent (0.15M DtBu18C6and 1.2M TBP in the Isopar® L diluent) and of the solvent mixture (0.15MDtBu18C6 and 0.75M Cs-7SB modifier in the Isopar® L diluent); and

FIG. 6 shows the nitric acid dependency at ambient temperature(approximately 23° C.) of an embodiment of a mixed extractant solventthat includes DtBu18C6, BOBCalixC6, and Cs-7SB.

DETAILED DESCRIPTION OF THE INVENTION

A mixed extractant solvent for extracting cesium and strontium from anacidic solution is disclosed. The mixed extractant solventsimultaneously or concurrently extracts cesium and strontium from theacidic solution. The cesium and strontium are collectively referred toherein as “radionuclides.” The mixed extractant solvent includes a crownether compound, a calixarene compound, and at least one modifierdissolved in a diluent. The crown ether compound and the calixarenecompound are collectively referred to herein as “extractants.” The mixedextractant solvent may form a first organic phase of a first extractionsystem that also includes a first aqueous phase. The extractants may besufficiently soluble in the first organic phase so that a highconcentration of the extractants is achieved. The concentration of theextractants in the first organic phase may be sufficiently high toeffectively remove the radionuclides from the acidic solution. Theextractants may also be relatively insoluble in the first aqueous phase.

The crown ether used in the mixed extractant solvent may be4′,4′,(5′)-di-(t-butyldicyclo-hexano)-18-crown-6 (“DtBul18C6”). DtBu18C6is available from Eichrom Industries Inc. (Darien, Ill.) and has amolecular weight of 484.72 g/mol. The crown ether may be present in themixed extractant solvent from approximately 0.001M to approximately0.5M. In one embodiment, the crown ether is present from approximately0.086 M to approximately 0.108 M. DtBu18C6 has the following structure:

The calixarene used in the mixed extractant solvent may becalix[4]arene-bis-(tert-octylbenzo)-crown-6 (“BOBCalixC6”). BOBCalixC6is available from IBC Advanced Technologies, Inc. (American Fork, Utah)and has a molecular weight of 1149.52 g/mol. The calixarene may bepresent in the mixed extractant solvent from approximately 0.0025M toapproximately 0.025M. BOBCalixC6 has the following structure:

The modifier may be an alcohol modifier, trioctylamine (“TOA”),tri-n-butyl phosphate (“TBP”), or mixtures thereof. The modifier mayincrease the extractants' ability to extract the radionuclides and mayenable a lower concentration of the extractants to be used in the mixedextractant solvent. Since many crown ether and calixarene compounds havelimited solubility in diluents, the modifier may keep the extractantsdissolved in the diluent. The modifier may also prevent the formation ofa third phase during the extraction. In addition, the modifier mayimprove stripping efficiency of the radionuclides, enabling the cesiumand strontium to be effectively removed or stripped from the mixedextractant solvent. The alcohol modifier may have a general structure of

where R and R′ are as described in Leonard et al., Sep. Sci. andTechnol., 36(5-6):743-766 (2001), Leonard et al., Solvent Extr. And IonExch., 21(4):505-526 (2003), and Duchemin et al., Solvent Extr. And IonExch., 19(6):1037-1058 (2001). For instance, the alcohol modifier may be1-(2,2,3,3-tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol(“Cs-7SB”), which has the following structure:

The Cs-7SB may be present in the mixed extractant solvent fromapproximately 0.2M to approximately 1.0M.

The diluent may be an inert diluent, such as a straight chainhydrocarbon diluent. For instance, the diluent may be an isoparaffinichydrocarbon diluent, such as Isopar® L or Isopar® M. Isopar® L includesa mixture of C₁₀-C₁₂ isoparaffinic hydrocarbons and is available fromExxon Chemical Co. (Houston, Tex.). Isopar® M includes a mixture ofC₁₂-C₁₅ isoparaffinic hydrocarbons and is available from Exxon ChemicalCo. (Houston, Tex).

The mixed extractant solvent may include other combinations of cesiumextractants and strontium extractants besides DtBu18C6 and BOBCalixC6.For instance, combinations of other crown ethers and calixarenes thatare capable of concurrently extracting cesium and strontium may be used.In general, crown ethers having a dicyclohexano structure may provideselectivity for strontium and those having a dibenzo structure mayprovide selectivity for cesium. Additional crown ethers are known in theart and include, but are not limited to, cis-dicyclohexano-18-crown-6(“DCH18C6”), dimethyl derivatives thereof, and di-t-butyl derivativesthereof. Additional calixarenes are known in the art and may be used inthe mixed extractant solvent, such as derivatives ofcalix[4]arene-crown-6 ether including, but not limited to, mono- andbis-crown-6-derivatives of 1,3 calix[4]arenes. The calixarenes may be incone, partial cone, 1,2 alternate, or 1,3 alternate conformations. Themixed extractant solvent may also include other alcohol modifiers, suchas derivatives of 2-(4-tert-octylphenoxy)-1-ethanol or derivatives of1-(4-tert-octylphenoxy)-3-(1,1,2,2-tetrafluoroethoxy)-2-propanol. Thealcohol modifiers may have fluorine containing substituents on thealcohol carbon. In addition, other diluents, such as 1-octanol, may beused.

In one embodiment, the mixed extractant solvent includes 0.15M DtBu18C6,0.007M BOBCalixC6, and 0.75M Cs-7SB modifier in Isopar® L. The mixedextractant solvent may extract cesium and strontium from a 1M nitricacid solution with a D_(Sr) of approximately 10 and a D_(Cs) ofapproximately 8 at ambient temperature. In contrast, the SREX solventused alone had a substantially lower D_(Sr) of 0.7 while the CSSXsolvent used alone had a similar D_(Cs) of 8.0. The mixed extractantsolvent may provide improved cesium and strontium extraction compared toa 1:1 volume ratio of the SREX and CSSX solvents. When the SREX and CSSXsolvents were mixed in a 1:1 volume ratio, the D_(Sr) decreased to 1.5and the D_(Cs) dropped significantly to 0.64. These results indicatethat the mixed extractant solvent may provide substantially improvedextraction of the cesium and strontium compared to using the SREX andCSSX solvents alone or in a 1:1 volume ratio.

The distribution of cesium and strontium between the organic phase andthe aqueous phase may be determined by conventional techniques. Thedistribution ratio for strontium (“D_(Sr)”) was calculated as the ratioof organic phase activity to the aqueous phase activity at equilibrium.High values for the D_(Sr) indicate that the strontium is present in theorganic phase while low values for the D_(Sr) indicate that thestrontium is present the aqueous phase. Similarly, the distributionratio for cesium (“D_(Cs)”) was calculated as the ratio of organic phaseactivity to the aqueous phase activity at equilibrium. High values forthe D_(Cs) indicate that the cesium is present in the organic phasewhile low values for the D_(Cs) indicate that the cesium is present theaqueous phase.

The mixed extractant solvent may be prepared by combining the crownether, the calixarene, and the modifier with the diluent to form amixture. Initially, a portion of a final volume of the diluent may beadded to the extractants and the modifier to lower the viscosity of themixture. The mixture may be stirred overnight and the remainder of thediluent may then be added.

The mixed extractant solvent may be used to simultaneously extractcesium and strontium from the acidic solution, such as from an acidicnuclear waste solution. The acidic solution may include fromapproximately 0.001M to approximately 3M nitric acid (“HNO₃”). Sincethese nitric acid levels are similar to the levels typically present indissolved spent nuclear fuel, the mixed extractant solvent may be usedto effectively remove cesium and strontium from dissolved spent nuclearfuel solutions. For instance, the mixed extractant solvent may removecesium and strontium from an acidic solution having from approximately0.5M to approximately 3M nitric acid. In one embodiment, the mixedextractant solvent simultaneously extracts cesium and strontium from a1M nitric acid solution with a D_(Sr) of approximately 10 and a D_(Cs)of approximately 8 at ambient temperature. The mixed extractant solventmay remove substantially all of the cesium and strontium from the acidicsolution after four sequential extractions.

By removing the radionuclides, the mixed extractant solvent may be usedto lower the radioactive waste volume and heat load of the acidicsolution. In addition, the radionuclides and the mixed extractantsolvent may be recovered and the mixed extractant solvent may berecycled. The extraction method of the present invention may alsoproduce less secondary waste than in conventional techniques.Furthermore, since the cesium and strontium may be removedsimultaneously, the extraction system of the present invention may beadvantageous over conventional techniques, which require multiple,separate steps to remove the cesium and strontium.

As discussed in detail below, the mixed extractant solvent may provideimproved levels of extraction of cesium and strontium compared to thelevel of extraction achieved when the SREX and CSSX solvents arecombined. In other words, the mixed extractant solvent providessynergistic results for the removal of the strontium while coextractingthe cesium.

The mixed extractant solvent may be used to selectively extract cesiumand strontium over additional components in the acidic solution. Inaddition to cesium and strontium, the acidic solution may include otherions or radioactive elements. Typical components of dissolved spentnuclear fuel solutions are shown in Table 1. Simulants having variouscombinations of the components shown in Table 1 may be prepared to testthe mixed extractant solvent. TABLE 1 Typical major components ofdissolved, high burn-up spent nuclear fuel solutions. Component AmountComponent Amount Acid (M) 0.8 Pr g/l 0.63 Tc g/l 0.41 Nd g/l 2.34 Ba g/l1.59 Zr g/l 0.42 Ce g/l 1.37 Sm g/l 0.47 Cs g/l 1.43 Np g/l 0.43 La g/l0.70 Pu g/l 4.76 Pd g/l 1.03 Am g/l 0.62 Mo g/l 2.09 Sn g/l 1.39 Sr g/l0.44 Rb g/l 0.20 Pd g/l 1.03 Rb g/l 0.20

The cesium and strontium may be removed or forward extracted from theacidic solution by mixing the acidic solution with the mixed extractantsolvent. As used herein the terms “forward extract,” “forwardextracted,” or “forward extraction” refer to removing or extracting thecesium and strontium from the first aqueous phase of the firstextraction system. As such, the first extraction system may include theacidic solution (the first aqueous phase) and the mixed extractantsolvent (the first organic phase). The first organic phase and the firstaqueous phase may be agitated with one another to extract the cesium andstrontium into the first organic phase. The distribution of the cesiumand strontium between the first organic phase and the first aqueousphase may heavily favor the first organic phase. The acidic solution maybe mixed with the mixed extractant solvent for an amount of timesufficient to form complexes between the cesium and strontium and theextractants. For instance, complexes may be formed between the cesiumand the calixarene and between the strontium and the crown ether. Aftermixing the mixed extractant solvent and the acidic solution for anamount of time sufficient for the complexes to form, two phases may beformed in the first extraction system: the first organic phase and thefirst aqueous phase. The cesium and strontium may be present in thefirst organic phase while the first aqueous phase may be substantiallydepleted of cesium and strontium. The first aqueous phase may includeany other ions or radioactive elements that were present in the acidicsolution. The first organic phase and the first aqueous phase may thenbe separated, effectively removing the cesium and strontium from theacidic solution.

Once separated, the first organic phase and the first aqueous phase maybe further processed. For instance, the first aqueous phase may beextracted multiple times with the mixed extractant solvent to removesubstantially all of the cesium and strontium. The first aqueous phasemay also be further extracted to remove the additional ions orradioactive elements that may have been present in the acidic solution,such as by using conventional techniques. The radionuclides may bestripped or back extracted from the first organic phase to recover thecesium, strontium, and the mixed extractant solvent. As used herein, theterms “back extract,” “back extracted,” or “back extraction” refer toremoving or extracting the cesium and strontium from the mixedextractant solvent. During recovery and recycling conditions, thedistribution of the cesium and strontium between the first organic phaseand a second aqueous phase may heavily favor the second aqueous phase.The cesium and strontium may be removed from the first organic phase bycontacting the first organic phase with the second aqueous phase. Thesecond aqueous phase and the first organic phase may form a secondextraction system. The second aqueous phase may be a dilute acidicsolution, such as a nitric acid solution having from approximately0.001M HNO₃ to approximately 0.5M HNO₃. In addition, water or otherdilute mineral acids may be used as the second aqueous phase.

The first organic phase may be mixed with the second aqueous phase foran amount of time sufficient for the cesium and strontium ions todissociate from the complexes of the cesium and strontium with theextractants. Once dissociated, the cesium and strontium may be extractedinto the second aqueous phase. The second aqueous phase, havingsubstantially all of the cesium and strontium, may be separated from thefirst organic phase, which is substantially depleted of cesium andstrontium. The radionuclides in the second aqueous phase may then beused or stored. For instance, the cesium and strontium may be solidifiedfor storage. Alternatively, the recovered cesium and strontium may beused as gamma sources, beta sources, or heat sources. The recoveredmixed extractant solvent may be reused or recycled into subsequentextractions.

The acidic solution may also be processed to remove the additional ionsand radioactive elements before the cesium and strontium are removed bythe method of the present invention. The additional ions and radioactiveelements may be removed by exposure to conventional extractionprocesses.

The extraction and recovery of the cesium and strontium may be performedat a temperature ranging from approximately 1° C. to approximately 40°C. To provide optimal extraction of the cesium and strontium, theforward extraction may be conducted at low temperatures within thisrange, such as at a temperature ranging from approximately 10° C. toapproximately 15° C. However, the forward extraction may also beconducted at ambient temperature, such as from approximately 20° C. toapproximately 25° C. The backward extraction of the cesium and strontiummay be conducted at a wider range of temperatures, such as fromapproximately 10° C. to approximately 60° C. For instance, the backwardextraction may be performed at a temperature ranging from approximately20° C. to approximately 40° C.

A solvent mixture having the DtBu18C6 extractant, the Cs-7sB modifier,and a diluent may also be used to extract strontium from an acidicsolution. The solvent mixture may improve the forward distribution ofstrontium. For instance, the D_(Sr) from a 1M nitric acid solution maybe increased from approximately 0.7 using the SREX solvent to a range ofapproximately 5 to approximately 7 using the solvent mixture havingDtBu18C6, Cs-7sB, and the diluent.

The following examples serve to explain embodiments of the presentinvention in more detail. These examples are not to be construed asbeing exhaustive or exclusive as to the scope of this invention.

EXAMPLES

All solvents used in the extraction process were reagent grade and wereused as received. Deionized water was used to prepare all aqueous acidsolutions. The nitric acid was reagent grade and was obtained fromSigma-Aldrich Chemical Co. (St. Louis, Mo.). Isopar® L isoparaffinicdiluent was obtained from Exxon Chemical Co. (Houston, Tex.). The ⁸⁵Srand ¹³⁷Cs radiotracers used to spike the simulants were obtained as⁸⁵SrCl₂ in 1M HCl and ¹³⁷CsCl in 1M HCL from Isotope Products (Burbank,Calif.). Both radiotracers were converted to the nitrate form prior touse. The DtBu18C6 was purchased from Eichrom Industries Inc. (Darien,Ill.). The BOBCalixC6 and the Cs-7SB modifier were obtained from OakRidge National Laboratory and were used as received.

Comparative Example 1 Preparation of the SREX Solvent and the CSSXSolvent

The SREX solvent was prepared using a mixture of 0.1 5M DtBu18C6, 1.2MTBP, and Isopar® L as described in Horowitz et al., Solvent Extr. AndIon Exch., 9(1):1-25 (1991). The mixture was stirred for approximately 1hour, until the DtBu18C6 and TBP went into solution.

The CSSX solvent included 0.007M BOBCalixC6, 0.750M Cs-7SB modifier,0.003M TOA, and Isopar® L as described in Bonnesen et al., “Extractionof Cesium from Savannah River Tank Waste Using a Calixarene Crown EtherExtractant,” Report ORNL/TM-13704, Oak Ridge National Laboratory: OakRidge, Tenn. (December 1998) and was received already prepared.

Comparative Example 2 Nitric Acid Dependencies of the SREX Solvent andthe CSSX Solvent

The ⁸⁵Sr and ¹³⁷Cs radiotracers were diluted to 7.3 μCi/ml and heated toincipient dryness. Concentrated HNO₃ was then added to convert theradiotracers to the nitrate salts. After three such cycles, 10 ml ofvarying concentrations of HNO₃ (from 0.01M to 10M) were added to theradiotracers to redissolve the salts in preparation for the extractionstudies. Carrier free ⁸⁵Sr in varying concentrations of HNO₃ was mixedin equal proportions with the SREX solvent and shaken for 1 minute. Thesample was then centrifuged for 1 minute and the resulting organic andaqueous phases sampled for analysis. Aliquots of the organic and aqueousphases were γ-ray counted using a Princeton Gamma-Tech (“PGT”) detectorhaving a bias of ⁺3500V. As shown in FIG. 1, the SREX solvent had aD_(Sr) of 0.70 in 1M HNO₃ at ambient temperature. The mean of this data(N=3) has an experimental uncertainty of ±5% in the distribution ratioand is consistent with previous work. In all additional testing,carriers were used for both the strontium and cesium except when statedotherwise. The carrier concentrations included 0.001M Sr(NO₃)₂ and0.0001M CsNO₃.

During the initial nitric acid dependency testing, it was discoveredthat the 8M and 10M HNO₃ samples exhibited third phase formation whenmixed with the SREX solvent. Therefore, testing was performed todetermine the acidity at which a third phase started to occur.Concentrations of 1M, 2M, 3M, 4M, and 5M HNO₃ were tested and it wasobserved that the third phase formed in extraction contacts havinggreater than or equal to 3M HNO₃.

A D_(Cs) nitric acid dependency test was also performed for the CSSXsolvent, the results of which are shown in FIG. 2. The CSSX solvent hada D_(Cs) of 8.0 in 1M HNO₃ at ambient temperature. The D_(Cs) nitricacid dependency is linear and at unity with respect to the distributionratio when plotted on a log-log basis. The slopes of the nitric aciddependency for both cesium and strontium showed that Sr²⁺ was chargebalanced by 2NO₃ ⁻ and Cs⁺ was balanced by one NO₃ ⁻, which concurredwith previous work.

It was also determined that no coextraction of the cesium into the SREXsolvent and no extraction of the strontium into the CSSX solventoccurred when the SREX solvent and the CSSX solvent were evaluatedseparately.

Comparative Example 3 A 1:1 Volume Ratio of the SREX and CSSX SolventsDoes Not Effectively Remove Cesium and Strontium

When the SREX and CSSX solvents were mixed in a 1:1 volume ratio, theD_(Sr) increased slightly to 1.5 but the D_(Cs) dropped significantly to0.64. In contrast, the SREX solvent when used alone had a D_(Sr) of 0.70and the CSSX solvent when used alone had a D_(Cs) of 8.0, as describedin Comparative Example 2. Since the D_(Cs) dropped significantly, theseresults indicate that simply combining the SREX and the CSSX solvents ina 1:1 volume ratio did not effectively coextract both cesium andstrontium.

Example 1 Coextraction of Cesium and Strontium

Neat DtBu18C6, the extractant used in the SREX solvent, was added invarying concentrations to the CSSX solvent. Unexpectedly, an increasedforward distribution for strontium was observed. In fact, the D_(Sr)increased dramatically to 9.8 while the D_(Cs) remained approximatelythe same (D_(Cs)=8.0) as obtained with the CSSX solvent alone. To attainhigh distribution coefficients for both cesium and strontium,simultaneously, an optimum mixture for the DtBu18C6 and the BOBCalixC6was found. A plot of the cesium and strontium distribution coefficientsas a function of the ratios of DtBu18C6 to 0.007M BOBCalixC6 is shown inFIG. 3. Distribution coefficients for the cesium and strontium divergedat high and low concentrations of the DtBu18C6. However, favorableforward extraction of the cesium and strontium was obtained at DtBu18C6concentrations ranging from 0.053M to 0.378M. The distributioncoefficients were almost equal at DtBu18C6 concentrations ranging from0.086 M to 0.108 M. At these DtBu18C6 concentrations, the mixedextractant solvent extracted approximately six times more cesium andstrontium in a single contact than remained in the nitric acid solution.This level of extraction provides a cesium and strontium removal of99.9% in four sequential contacts with the mixed extractant solvent.

In order to determine which component or components of the mixedextractant solvent were causing the synergy, the two main componentsused in the CSSX solvent (BOBCalixC6 and Cs-7SB modifier) and theDtBu18C6 from the SREX solvent were procured. Different concentrationsof these components in various combinations were mixed and tests wereconducted on the different variations.

Since it was determined that 1M HNO₃ gave favorable combined forwarddistributions for cesium and strontium, 1M HNO₃ was used as the aqueousphase acidity unless otherwise specified. All solvents used werepreequilibrated with 1M HNO₃. The first combination of tested solventsincluded 0.15M DtBu18C6 and 0.75M Cs-7SB modifier in Isopar®L. In thissolvent extraction system, the D_(Sr) was 6.36 and the D_(Cs) was 0.046at 24° C. Since the D_(Cs) value was low, these results indicated thatno cesium extraction occurred with the DtBu18C6 but that enhancedstrontium extraction occurred due to the Cs-7SB modifier.

The second combination of solvents included 0.15M DtBu18C6 and 0.007MBOBCalixC6, which were the same concentrations used in the CSSX and SREXsolvents, while the Cs-7SB modifier concentration was varied. Theresults of these extractions are shown below in Table 2. TABLE 2 Cesiumand Strontium Distribution Ratios as a Function of Varying Cs-7SBConcentration. Concentration Of Cs-7SB (M) D_(Cs) D_(Sr) 0.01 7.7E−30.01 0.10 0.17 0.23 0.75 3.51 4.04 0.80 3.80 1.92The data in Table 2 indicated that although the D_(Cs) increased withincreasing Cs-7SB concentration, the D_(Sr) peaked at or near aconcentration of 0.75M of the Cs-7SB. Thus, for the remainder of theextraction tests, the concentration of the Cs-7SB remained at 0.75M.

An extraction of strontium with DtBu18C6, TBP, and TOA in Isopar® L wasperformed as well as an extraction of DtBu18C6 and TOA in Isopar® L. TheD_(Sr) was 0.56 and 0.01 at 20° C., respectively. These results, coupledwith the results from the mixture of SREX and CSSX described earlier,indicated that the TBP could be removed from the mixed extractantsolvent. The results also indicated that the TOA was optional under theexperimental extraction conditions. The distribution ratios showed thatwhen the TOA was added to the SREX solvent, it extracted strontium nobetter than in the SREX solvent alone. A study also determined thatusing a solvent including only the Cs-7SB modifier in Isopar® Lexhibited no extraction of cesium and strontium from 1 M nitric acidsolutions.

While the TBP of the SREX solvent was originally used in the mixedextractant solvent to enhance the solubility of the strontium in theorganic phase, the TBP was found to provide no additional benefit to thecoextraction of the cesium and strontium with the mixed extractantsolvent. Rather, it was determined that the TBP possibly hinderedforward strontium extraction. The TOA, which was added to the CSSXprocess to aid in cesium stripping by preventing undesired complexationof cesium with organic impurities dissolved during continuousprocessing, did not interfere with strontium forward distributions inthe tests using neat DtBu18C6 added to the CSSX solvent. However, TOAwas not used in the mixed extractant solvent so that the testing couldbe performed under controlled conditions. Therefore, when thisextraction method is employed on an industrial scale, it may benecessary to add TOA to the mixed extractant solvent.

Example 2 Extraction of Strontium with Cs-7SB

Tests conducted to determine the component responsible for the elevatedstrontium distributions indicated that the Cs-7SB modifier provided theincreased strontium distribution. The forward distribution of strontiumfrom 1M HNO₃ solutions was increased from 0.7 at ambient temperatureusing SREX (DtBu18C6 and TBP in Isopar®L) to between 5 and 7 using amixture of DtBu18C6, the Cs-7SB modifier, and the Isopar L ® diluent. Assuch, the Cs-7SB modifier provided a significant improvement over theSREX extractant for processes where selective strontium removal fromacidic solutions is desired. While the positive effect of fluorinatedmodifiers is known for the extraction of cesium using crown ethers orcalixarenes, these modifiers have not been used instead of TBP in theSREX solvent for enhancing strontium extraction.

Example 3 Temperature Dependence of the Cesium/Strontium Extraction

When a second round of testing was performed to reproduce the datapresented in FIG. 3, the ambient temperature had dropped from 24° C. to20° C. At 20° C., the D_(Sr) from a 1M HNO₃ solution was found to behigher, with a D_(Sr) of 11.3 instead of a D_(Sr) of 9.8 at 24° C. (asdescribed in Example 1). Since high distribution ratios are desired, theincrease in D_(Sr) with decreasing temperature was of interest since ithas been noted in the literature that the forward distributions ofcesium in BOBCalixC6 are similarly temperature dependent. To furtherelucidate the effect of temperature, temperature dependence tests wereperformed on the SREX solvent, the CSSX solvent, and the mixedextractant solvent. Tests performed at 10° C. showed that the D_(Sr) forthe SREX solvent was 2.2 and the D_(Cs) for the CSSX solvent was 46.0.In contrast, the distribution ratios at 24° C. were lower, with a D_(Sr)of 0.70 and a D_(Cs) of 8.0. The temperature dependence tests on themixed extractant solvent showed that very favorable distributions wereachieved at 10° C., as shown in FIG. 4. The D_(Sr) and D_(Cs) with themixed extractant solvent were approximately 16 and approximately 20,respectively. The non-linear shape seen in FIG. 4 is due to an enthalpyeffect, which indicates that this is an exothermic and, thus, favorablereaction. The lower the temperature, the more excess energy is removedfrom the system, driving the equilibrium and raising the forwarddistributions of the cesium and strontium.

Example 4 Nitrate Dependency Tests on Strontium Distributions from theSREX Solvent and the Solvent Mixture

Nitrate dependency tests were performed on the SREX solvent (0.15MDtBu18C6 and 1.2M TBP in the Isopar L diluent) and on the solventmixture (0.15M DtBu18C6 and 0.75M Cs-7SB modifier in the Isopar Ldiluent) using Al(NO₃)₃ in an effort to compare and determine the effectof nitrate ions on the forward distribution of strontium. The nitrateconcentrations were varied by adding Al(NO₃)₃ to 0.5M HNO₃ to yield0.6M, 0.7M, 0.9M, 1.0M, and 2.5M total NO₃. The results from these testsare shown in FIG. 5 and indicate an increase in strontium distributionwith increasing nitrate concentrations, giving a slope of approximately2 for both solvents. These slopes indicate a charge balance of the Sr²⁺with 2NO₃ ⁻ ions, which concurs with previous work. In addition, thedata in FIG. 5 shows that the solvent mixture (DtBu18C6 and Cs-7SBmodifier in Isopar® L) gives a factor of 4 higher distribution than theSREX solvent. Concentrations of 2.5M NO₃ yielded a forward distributionfor the solvent mixture (DtBu18C6 and Cs-7SB modifier in Isopar L) ofD_(Sr)=46, while the distribution for the SREX solvent was D_(Sr)=12 atambient temperature.

Example 5 Recovery of Cesium and Strontium using the Mixed ExtractantSolvent

To develop a complete extraction process, the ability to sequentiallyextract and back extract the cesium and strontium and to recover andreuse the mixed extractant solvent was studied. A combination ofsolvents that included 0.15M DtBu18C6, 0.007M BOBCalixC6, and 0.75MCs-7SB modifier in Isopar®L was prepared by adding neat DtBu18C6,BOBCalixC6, and the Cs-7SB modifier to a mixing vessel. Approximately10% of the required final Isopar®L volume was added to lower theviscosity and the mixture was left to stir overnight. The remainder ofthe Isopar®L was then added as a diluent the next morning to form themixed extractant solvent.

Sequential extractions of fresh aliquots of the mixed extractant solventwith the same aliquot of radioactive traced carrier solution gave thefollowing forward distribution ratios for cesium and strontium: TABLE 3Forward Distribution Ratios for Cesium and Strontium. Contact # D_(Cs)D_(Sr) 1 8.1 9.5 2 11.1  11.6  3 7.6 8.4 4 * ** The level of cesium and strontium remaining in the acidic solutionwere below detection limits in extraction contact #4.

Efficient stripping of most of the cesium and strontium from the organicphase was demonstrated in two stages by repeatedly contacting equalvolumes of the organic phase with 0.01M HNO₃. The results of two typicalback extractions (strips) at 20° C. are shown in Table 4. TABLE 4Backward Distribution Ratios for Cesium and Strontium. D_(Cs) D_(Sr)Strip #1 0.33 0.02 Strip #2 0.17 0

Example 6 Nitric Acid Dependency of the Mixed Extractant Solvent

Nitric acid dependency tests were performed at ambient temperature(approximately 23° C.) on the mixed extractant solvent described inExample 5. Varying concentrations of HNO₃, from 0.01M to 2M, were mixedin equal proportions with the mixed extractant solvent. The nitric aciddependency test was performed in a manner similar to that described inComparative Example 2. The results of the nitric acid dependency areshown in FIG. 6 and Table 5. TABLE 5 Forward Distribution Ratios forCesium and Strontium in Varied HNO₃ Concentrations. HNO₃ Concentration(M) D_(Cs) D_(Sr) 0.01 0.19 0.01 0.1 1.14 0.23 0.5 3.44 1.98 1.0 5.667.37 2.0 9.26 20.8

The crossover point, the molarity at which both the forward distributionof cesium and strontium is equal to or greater than approximately 1, isapproximately 0.3M. The forward distribution of cesium is equal to orgreater than approximately 1 at a lower molarity of 0.1 M.

In summary, the mixed extractant solvent has been shown tosimultaneously remove cesium and strontium from acidic solutions. Thedistribution ratios (D_(Cs) and D_(Sr)) achieved using the mixedextractant solvent are significantly higher than the distribution ratiosof the SREX solvent or the CSSX solvent used alone or in a 1:1 volumeratio.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

1. A mixed extractant solvent for extracting cesium and strontium,comprising: calix[4]arene-bis-(tert-octylbenzo)-crown-6 (“BOBCalixC6”),4′,4′,(5′)-di-(t-butyldicyclo-hexano)-18-crown-6 (“DtBu18C6”), and atleast one modifier dissolved in a diluent.
 2. The mixed extractantsolvent of claim 1, wherein the BOBCalixC6 is present from approximately0.0025M to approximately 0.025M.
 3. The mixed extractant solvent ofclaim 1, wherein the DtBu18C6 is present from approximately 0.01M toapproximately 0.5M.
 4. The mixed extractant solvent of claim 1, whereinthe DtBu18C6 is present from approximately 0.086 M to approximately0.108 M.
 5. The mixed extractant solvent of claim 1, wherein the atleast one modifier is1-(2,2,3,3-tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol(“Cs-7SB”).
 6. The mixed extractant solvent of claim 1, wherein the atleast one modifier is present from approximately 0.2M to approximately1.0M.
 7. The mixed extractant solvent of claim 1, wherein the diluent isan isoparaffinic hydrocarbon.
 8. The mixed extractant solvent of claim1, wherein the mixed extractant solvent comprises approximately 0.15MDtBu18C6, approximately 0.007M BOBCalixC6, and approximately 0.75MCs-7SB modifier dissolved in an isoparaffinic hydrocarbon.
 9. The mixedextractant solvent of claim 1, further comprising trioctylamine,tri-n-butyl phosphate, or mixtures thereof.
 10. An extraction system forextracting cesium and strontium, comprising: an organic phase comprisinga mixed extractant solvent havingcalix[4]arene-bis-(tert-octylbenzo)-crown-6 (“BOBCalixC6”),4′,4′,(5′)-di-(t-butyldicyclo-hexano)-18-crown-6 (“DtBu18C6”), and atleast one modifier dissolved in a diluent; and an aqueous phasecomprising an acidic solution including cesium and strontium.
 11. Theextraction system of claim 10, wherein the BOBCalixC6 is present in themixed extractant solvent from approximately 0.0025M to approximately0.025M.
 12. The extraction system of claim 10, wherein the DtBu18C6 ispresent in the mixed solvent extractant from approximately 0.01M toapproximately 0.5M.
 13. The extraction system of claim 10, wherein theDtBu18C6 is present in the mixed extractant solvent from approximately0.086 M to approximately 0.108 M.
 14. The extraction system of claim 10,wherein at least one modifier is1-(2,2,3,3-tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol(“Cs-7SB”).
 15. The extraction system of claim 10, wherein at least onemodifier is present in the mixed extractant solvent from approximately0.2M to approximately 1.0M.
 16. The extraction system of claim 10,wherein the diluent is an isoparaffinic hydrocarbon.
 17. The extractionsystem of claim 10, wherein the mixed extractant solvent comprisesapproximately 0.15M DtBu18C6, approximately 0.007M BOBCalixC6, andapproximately 0.75M Cs-7SB modifier dissolved in an isoparaffinichydrocarbon.
 18. The extraction system of claim 10, wherein the mixedextractant solvent further comprises trioctylamine, tri-n-butylphosphate, or mixtures thereof.
 19. The extraction system of claim 10,wherein the acidic solution comprises an aqueous solution having fromapproximately 0.5M to approximately 3M nitric acid.
 20. The extractionsystem of claim 10, wherein the acidic solution comprises a dissolvedspent nuclear fuel including cesium and strontium.
 21. A method ofseparating cesium and strontium from an acidic solution, comprising:providing an acidic solution comprising cesium and strontium; contactingthe acidic solution with a mixed extractant solvent comprisingcalix[4]arene-bis-(tert-octylbenzo)-crown-6 (“BOBCalixC6”),4′,4′,(5′)-di-(t-butyldicyclo-hexano)-18-crown-6 (“DtBu18C6”), and atleast one modifier dissolved in a diluent; and removing the cesium andstrontium from the acidic solution.
 22. The method of claim 21, whereinproviding an acidic solution comprising cesium and strontium comprisesproviding the acidic solution comprising from approximately 0.5M toapproximately 3M nitric acid.
 23. The method of claim 21, whereincontacting the acidic solution with a mixed solvent extractant comprisescontacting the acidic solution with the mixed extractant solventcomprising from approximately 0.0025M to approximately 0.025MBOBCalixC6.
 24. The method of claim 21, wherein contacting the acidicsolution with a mixed solvent extractant comprises contacting the acidicsolution with the mixed extractant solvent comprising from approximately0.01M to approximately 0.5M DtBu18C6.
 25. The method of claim 21,wherein contacting the acidic solution with a mixed solvent extractantcomprises contacting the acidic solution with the mixed extractantsolvent comprising from approximately 0.086 M to approximately 0.108 MDtBu18C6.
 26. The method of claim 21, wherein contacting the acidicsolution with a mixed solvent extractant comprises contacting the acidicsolution with the mixed extractant solvent comprising at least onemodifier that includes1-(2,2,3,3-tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol(“Cs-7SB”).
 27. The method of claim 26, wherein contacting the acidicsolution with a mixed solvent extractant comprises contacting the acidicsolution with the mixed extractant solvent comprising from approximately0.2M to approximately 1.0M Cs-7SB.
 28. The method of claim 21, whereincontacting the acidic solution with a mixed solvent extractant comprisescontacting the acidic solution with the mixed extractant solventcomprising a diluent that includes an isoparaffinic hydrocarbon.
 29. Themethod of claim 21, wherein contacting the acidic solution with a mixedsolvent extractant comprises contacting the acidic solution with themixed extractant solvent comprising approximately 0.15M DtBu18C6,approximately 0.007M BOBCalixC6, and approximately 0.75M Cs-7SB modifierdissolved in an isoparaffinic hydrocarbon diluent.
 30. The method ofclaim 21, wherein contacting the acidic solution with a mixed solventextractant comprises contacting the acidic solution with the mixedextractant solvent that further comprises trioctylamine, tri-n-butylphosphate, or mixtures thereof as the at least one modifier.
 31. Themethod of claim 21, wherein contacting the acidic solution with a mixedextractant solvent comprises forming a first organic phase and a firstaqueous phase.
 32. The method of claim 21, wherein contacting the acidicsolution with a mixed extractant solvent comprises extracting the cesiumand strontium into a first organic phase.
 33. The method of claim 21,wherein removing the cesium and strontium from the acidic solutioncomprises separating a first organic phase and a first aqueous phase.34. The method of claim 21, wherein removing the cesium and strontiumfrom the acidic solution comprises removing the cesium and strontium ata temperature ranging from approximately 1° C. to approximately 40° C.35. The method of claim 21, wherein removing the cesium and strontiumfrom the acidic solution comprises removing the cesium and strontium ata temperature ranging from approximately 10° C. to approximately 15° C.36. The method of claim 21, further comprising recovering the mixedextractant solvent, the cesium, and the strontium.
 37. The method ofclaim 36, wherein recovering the mixed extractant solvent, the cesium,and the strontium comprises contacting a first organic phase with asecond aqueous phase.
 38. The method of claim 37, wherein contacting afirst organic phase with a second aqueous phase comprises extracting thecesium and strontium into the second aqueous phase.
 39. The method ofclaim 37, wherein contacting a first organic phase with a second aqueousphase comprises contacting the first organic phase with the secondaqueous phase at a temperature ranging from approximately 10° C. toapproximately 60° C.
 40. The method of claim 37, wherein contacting afirst organic phase with a second aqueous phase comprises contacting thefirst organic phase with the second aqueous phase at a temperatureranging from approximately 20° C. to approximately 40° C.
 41. The methodof claim 37, wherein contacting a first organic phase with a secondaqueous phase comprises contacting the first organic phase with anaqueous solution comprising from approximately 0.001M nitric acid toapproximately 0.5M nitric acid.
 42. The method of claim 36, whereinrecovering the mixed extractant solvent, the cesium, and the strontiumcomprises separating a first organic phase and a second aqueous phase.43. A method of extracting strontium, comprising: contacting an acidicsolution comprising strontium with a solvent comprising4′,4′,(5′)-di-(t-butyldicyclo-hexano)-18-crown-6 (“DtBu18C6”),1-(2,2,3,3-tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol(“Cs-7SB”), and an isoparaffinic hydrocarbon.